Predictions of 13C chemical shifts in carbocations. The use of scaled chemical shifts calculated using GIAO DFT methods†

Abstract

Accurate prediction of ¹³C chemical shifts (δ_pred) for the C⁺ and the α- and β-carbon atoms in alkyl and cycloalkyl carbocations is achieved through scaling. Abinitio calculated chemical shifts (δ_calc) at the GIAO-B3LYP/6-311G(d,p)//B3LYP/6-31G(d) level of theory for 16 different carbocations are scaled using a linear correlation equation (δ_pred = aδ_calc + b). The slope and intercept a and b were determined separately for C⁺ and the α- and β-carbon positions, and were found to be different for cation structures preferentially stabilized by β-C–H and β-C–C hyperconjugation. A very good correlation of all predicted and experimental chemical shifts is obtained (a = 0.999 ± 0.002, b = 0.231 ± 0.320). Preliminary results indicate that scaling using linear correlations can also be applied to the C⁺ carbon chemical shift in benzyl type carbocations.